Concrete in Australia Vol 40 No 3 31
shrinkage, particularly the difference in creep and shrinkage
effects between the eastern and western carriageways.
One significant effect was the offset horizontal centroid of the
sections, which varied from 400 mm for the typical segments
up to 800 mm for some segments in the bifurcation area.
This resulted in rotated principal axes ranging from 2° to 8° to
the horizontal and stress variations of up to 8 MPa across the
width of the top slab. In extreme cases, non-symmetrical post
tensioning was utilised and not capturing these effects would
have led to an inadequate design.
Movement joints
To accommodate movements due to temperature, concrete
creep and shrinkage and to relieve loads in the superstructure
and substructure, expansion joints were required every four
to five spans. To enable balanced cantilever construction to
proceed without disruption, mid-span joints with needle
beams were adopted, the first time that this approach has been
used in Australia.
With expansion joints at mid-span, the steel needle beams
span across the joints and maintain the relative attitude of
the ends of the two cantilevers, while allowing longitudinal
expansion as shown in Figure 11. The needle beams are
supported on bearings at internal diaphragms in the girders.
The beams are placed inside the box girder after the completion
of the first cantilever, then pulled back into the girder. When
the second cantilever and the continuity stressing has been
completed, they are pulled back across the joint and secured in
place.
Moments in the needle beams cause equal loads in each of the
bearings but shear causes unequal loadings. Because the shear
at mid span is usually low this is not a significant issue, but
large torsions can be generated by live loads, in particular if live
loads are placed on the north eastern and south western sides
of a joint. This can create significant shears in the beams and
differences in the bearing loads of up to 12 MN.
To transfer the vertical loads from the bearings into the webs
of the girder very substantial diaphragms are required. The
bearings are supported on steel boxes cast into the diaphragms,
and these boxes have large reinforcement bars welded to them,
to carry the load up or down into the diaphragm itself. To
transfer this load into the webs without creating unacceptable
tensions in the diaphragm, additional reinforcement and draped
prestressing was installed to supplement the reinforcement
welded to the imbedded steel box. This post-tensioning runs
through the webs and bottom slab of the segment and is
anchored at the top, as shown in Figure 12.
Conclusions
The design and construction of the South Road Superway
project provided a number of interesting challenges to the
design and construction teams. The construction method
adopted demonstrated an innovative solution, which
overcame technical and geometric challenges and resulted in
an aesthetically pleasing viaduct with limited maintenance
required for its 150 year design life.
The authors would like to acknowledge the contributions
made by the entire design and construction team and the
assistance provided by DPTI. The innovations outlined in this
paper were a direct result of a successful design and construction
team relationship.
Figure 12: Needle beam diaphragm segment showing steel casing and draped prestress.
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